[OSM-talk] High-Precision GPS Survey Equipment?

[Charlie Echo]

Just to confirm:
GPS position is calculated from signals received from satellites: if all satellites give you the time and their position, by calculating the "difference of time", you get the "difference of distance to them", and consequently the position.

If the signals are reflected on walls, their "distance from satellite" may increase by 10 or 20 meters, or even more. So there is no way, in such conditions, to get a precise position, except by waiting to have all signals in direct view.
High precision GPS have highly accurate clocks, so that they can calculate precisely the difference of time. Possibly, they can decide to neglect some measurements out of range (if 5 satellites give one position and a 6th one a different position). 

But you will not get a meter-accuracy with GPS around buildings.

Another way to increase accuracy MAY BE to add an acceleration sensor, that will give you your position compared to one reference point (because a GPS in open-space can give you a good accuracy after some time).


----- Mail Original -----
De: "Ævar Arnfjörð Bjarmason" <avarab at gmail.com>
À: simon at mungewell.org
Cc: "osm" <talk at openstreetmap.org>
Envoyé: Lundi 25 Août 2008 19:22:17 GMT +01:00 Amsterdam / Berlin / Berne / Rome / Stockholm / Vienne
Objet: Re: [OSM-talk] High-Precision GPS Survey Equipment?

On Mon, Aug 25, 2008 at 4:02 PM,  <simon at mungewell.org> wrote:
> Hi Ian,
> How much precision do you want/need? And apart from the fun of it, does it
> actually benefit OSM going to these lengths?

My fairly accurate Garmin 60CSx is still wildly inaccurate when I
position myself at the corner of a building and try to measure its
dimensions since the building and the surrounding blocks most GPS
signals. So more accurate surveying equipment would for one thing be
more useful in getting data of regular quality in areas where there's
bad reception.

There's also the whole use case of mapping things with <5m accuracy
where most GPS units start being very inaccurate.

_______________________________________________
talk mailing list
talk at openstreetmap.org
http://lists.openstreetmap.org/listinfo/talk